The present invention relates to a new method for manufacturing varistors using nanocrystalline powders obtained by intensive milling.
It also relates to the so manufactured varistors, which differ from similar products presently available in particular in that they have a very high break-down voltage.
It has been known for a great numbers of years to use varistors containing zinc oxide to protect electrical equipments against over-voltages. Varistors are electrically active elements whose impedance varies in a non-linear manner as a function of the voltage applied to its terminals. These elements are usually in the form of pellets having a diameter of 3 to 100 mm and a thickness of 1 to 30 mm. They essentially consist of a material made of conducting grains of zinc oxide (ZnO) surrounded by insulating grain boundaries made of bismuth oxide (Bi2O3). After pressing, the pellets are subjected to sintering in a furnace at temperatures ranging from 1000 to 1500xc2x0 C. for several hours.
At low voltages, the insulating barriers at grain boundaries prevent the current from flowing. Therefore, the material acts as an insulator. When the voltage exceeds a given value called xe2x80x9cbreak down voltagexe2x80x9d, the resistance of the boundaries decreases rapidly, thereby making the material a variable resistance or xe2x80x9cvaristorxe2x80x9d. The material becomes then very conductive and the current can be diverted to the ground instead of damaging the electric equipment. Because of their structure, the varistors are mainly used in lightning-arresters like those of the electric energy transportation and distribution networks.
The lightning arresters presently available on the market usually comprise an insulating envelope in the form of a cylindrical tube. This envelope defines a cavity in which are mounted one or several columns of varistors packed one above the others. Each lighting arrester is connected in parallel to the electric equipment to be protected, in order to reduce the over-voltage that may be produced at the terminals of the same. From a practical standpoint, each lightning-arrester forms a normally open circuit which is xe2x80x9cconvertedxe2x80x9d into a closed circuit parallel to the equipment to be protected as soon as a significant over-voltage occurs at the terminals of the equipment. Such permits to reduce the insulation level of the electric equipment that is protected.
However, it is worth mentioning that there are numerous other potential applications for varistors, especially for the protection of secondary networks, domestic electric equipments, electronic or miniaturized equipments, etc.
Presently, there are numerous varistors available on the market, which are made of zinc oxide. By way of example of such varistors useful in lightning-arresters, reference can be made to those sold under the trademarks RAYCHEM and SEDIVER. These varistors are manufactured by sintering a mixture of powders of ZnO, Bi2O3 and, optionally, other oxides such as Sb2O3 and/or SiO2 at temperatures of about 1,200xc2x0 C. These varistors have an average grain size higher than 3 xcexcm (about 10 xcexcm for the RAYCHEM varistors and about 6 xcexcm for the SEDIVER varistors). Their break-down voltage is proportional to the numbers of grain boundaries or insulating barriers of Bi2O3 per unit length. Such break-down voltage is typically lower than 2.5 kV/cm (about 1.6 kV/cm for the RAYCHEM varistors and about 2 kV/cm for the SEDIVER varistors).
There are numerous scientific articles dealing with the structure and properties of ZnO-based varistors. Some of these articles suggest that the use of a pure or doped nanosize ZnO powder as a starting material would have numerous advantages, including, in particular, a substantial increase of their break-down voltage and of the coefficient of non-linearity of their current-voltage curve (hereinafter called xe2x80x9ccoefficient xcex1xe2x80x9d). Indeed, the break-down voltage seems to be inversely proportional to the ZnO grain size and, accordingly, to the sintering temperature.
By way of example of such articles, reference can be made to the following:
S. HINGORANI et al, xe2x80x9cMicroemulsion mediated synthesis of zinc-oxide nanoparticles for varistor studiesxe2x80x9d, Mat. Res. Bull., 28 (1993), 1303
S. HINGORANI et al, xe2x80x9cEffect of process variables on the grain growth and microstructure of ZnOxe2x80x94Bi2O3 varistors and their nanosize ZnO precursorsxe2x80x9d, J. of Materials Research, 10 (1995), 461;
J. LEE et al, xe2x80x9cImpedance spectroscopy of grain boundaries in nanophase ZnOxe2x80x9d, J. of Materials Research, 10 (1995) 2295;
R. N. VISWANATH et al, xe2x80x9cPreparation and characterization of nanocrystalline ZnO based materials for varistor applicationsxe2x80x9d, Nanostructured materials, 6 (1995), 993.
In these articles, nanoparticles of ZnO are prepared by microemulsion (see the articles of S. HINGORANI et al), by gaseous phase condensation (see the article of J. LEE et al) or by colloid suspension and centrifugal separation (see the article of R. N. VISWANATH et al). In all the cases, the obtained powder is pressed to form a pellet or a disc which is then subjected to sintering at a temperature which can be as low as 600xc2x0 C. to 750xc2x0 C. to avoid undue increase of the crystallite size (see the articles of R. N. VISWANATH et al and J. LEE et al) or as high as 1,200xc2x0 C. (see the articles of S. HINGORANI et al).
Recently, an article was published by the present inventors in the proceedings of ISMANAM-96. This article entitled xe2x80x9cBall milled ZnO for varistor applicationsxe2x80x9d, reports the result of tests carried out on pellets prepared from a nanocrystalline powder of pure ZnO obtained by intensive mechanical grinding and subsequently subjected to pressing and sintering at 1,250xc2x0 C. for 1 hour. These tests show that the so-obtained pellets have no varistor effects, contrary to those obtained from nanosize powder of ZnO obtained by gaseous phase condensation (see again the article of J. LEE et al).
In an article of Z. BRANKOVIC et al,  less than  less than Nanostructure constituents of ZnO-based varistors prepared by chemical attrition  greater than  greater than , Nanostructured Materials, 4 (1994), 149, there is disclosed a method for manufacturing a varistor comprising the following steps:
a) first preparing each of the main constituent phases of a ZnO-based varistor;
b) mixing together powders of the constituent phases;
c) intensively milling the powers after the mixing so that the obtained powders be nanocrystalline; and
d) submitting the so-milled mixture to a consolidation treatment comprising a pressing followed by a sintering at a temperature of 1,100xc2x0 C. (1,373xc2x0 K.) for 1 hour.
The final product that is so obtained, has the characteristics of a conventional varistor. The ZnO grain size ranges between 5.5 and 7.5 xcexcm (see Table 2), that is in the typical range of conventional varistors. Moreover, the break down voltages have a value comprised between 4.1 and 6.6 KV/cm. The author mentions: xe2x80x9cThere is no significant difference in electrical properties between the milled samples and sample Z1 (the reference sample) sintered under the same conditions, but the milled samples have higher values for the sintered density . . . It is evident that varistor mixtures which were intensively milled before sintering are more active for sintering process. It is the consequence of increase of surface free energy and defects concentration, as well as uniform distribution of powder particles and a decrease of powder particles sizexe2x80x9d.
U.S. Pat. No. 4,681,717 discloses a chemical process for manufacturing varistors, comprising the coprecipitation of metals followed by an oxidation by calcination and a sintering at a temperature of 675 to 740xc2x0 C. for periods exceeding 4 hours. The so-obtained varistors are disclosed as having a grain size lower than 1 xcexcm, a break-down voltage of 10 to 100 KV, a coefficient xcex1 of non-linearity higher than 30 and a density of about 65 to 99% of the theoretical density depending on the composition and the sintering temperature.
It has now been discovered that if:
on the one hand, use is made as starting materials of conventional or nanocrystalline powders obtained by intensive milling; and
on the other hand, the mixture obtained from these powders is subjected to an intense milling followed by a consolidation treatment including a sintering under such time and temperature conditions that ZnO keeps a grain size as low as possible;
one may obtain varistors having a very fine and homogeneous microstructure with an average grain size typically lower than or equal to 3 xcexcm, which is 3 to 5 times smaller than the grain size of conventional materials.
These new varistors have a higher numbers of grain boundaries per unit length and therefore a much higher break-down voltage. This break-down voltage is typically higher than 10 kV/cm and may be as high as 17 kV/cm, which is about one order of magnitude higher than the break-down voltage of conventional varistors. For a given operating voltage, such increase in performance permits, in principle, to reduce proportionally the size of the equipment protecting devices.
The coefficient xcex1 of non-linearity of the current-voltage curve is also substantially improved. It is higher than 20 and can reach value as high as 60 whereas it is of about 40 for the varistors of trademark SEDIVER and 36 for those of trademark RAYCHEM.
In addition, the leakage current below the break-down voltage of the varistors that are so manufactured, is smaller.
Accordingly, the first object of the present application is to provide a method for the manufacture of a varistor having a very high breakdown voltage, comprising the steps of:
(a) mixing powders of zinc oxide (ZnO) and bismuth oxide (Bi2O3) with at least one other powder of an additive capable of influencing the properties of varistors, said mixing being carried with such amounts of powders that the zinc oxide represents at least 75 mol % of the resulting mixture;
(b) subjecting the powders to an intensive milling before, during or after their mixing by means of a high energy ball mill in such a manner that the obtained powders be nanocrystalline; and
(c) subjecting the mixture of nanocrystalline powder that is so-obtained to a consolidation treatment,
characterized in that said consolidation treatment (c) includes a sintering and is carried out under time and temperature conditions selected to keep a zinc oxide grain size lower than 3 xcexcm and a low porosity.
Preferably, the intensive milling step (b) is carried out after the mixing step (a).
The powder of zinc oxide used a starting material can be milled before the mixing step (a), either alone or in combination with doping agents such as Al2O3. In parallel, the powder of bismuth oxide and all the other selected additives can be mixed, milled and treated at a high temperature equal to or higher than the one of step (c) before the mixing step (a).
Preferably also, the oxide powders or their mixture are calcinated at a temperature equal to or lower than 550xc2x0 C., before carrying out step (c) and before or after carrying out the intensive milling step (b), and the sintering made during the consolidation treatment of step (c) is carried out at a temperature lower than 1,200xc2x0 C. for a period of time equal to or lower than 2.5 hours. The heating rate to reach the sintering temperature is advantageously comprised between 0.5 and 1,200xc2x0 C./min and is preferably of about 1xc2x0 C./min.
Another object of the invention is to provide a varistor containing zinc oxide (ZnO) and bismuth oxide (Bi2O3), whenever obtained by the method disclosed hereinabove. This varistor has a very high break-down voltage, which is typically higher than 10 kV/cm, and numerous other interesting properties, including, in particular, a high coefficient ox of non-linearity of its current-voltage curve, and a small leakage-current. More precisely, the so manufactured varistor contains at least 75 mol % of ZnO and has the following characteristics:
it has a very fine and homogeneous microstructure with an average ZnO grain size lower than 3 microns;
it has a breakdown voltage higher than 10 kV/cm;
it has a coefficient of non-linearity of current-voltage higher than 20; and
it has a very small leak current below its breakdown voltage.
The varistors according to the invention are useful as protective elements for primary and secondary networks, electric equipments and electronic or miniaturized components. For example, they can be used for the manufacture of lightning arresters for the protection of transformers. They can also be used in electric outlets for protecting domestic electric equipments against over-voltages. They can further be used in micro-circuitry for protecting electronic components.
Thanks to their properties, and more particularly, to their high break-down voltage, the varistors according to the invention can be miniaturized, thereby permitting numerous applications that could not have been foreseen with conventional materials. Thus, for example, the conventional varistors have a relatively low break-down voltage (about 1.6 kV/cm for the varistors of trademark RAYCHEM). As a result, for an operative voltage of 30 kV, which is usually the one required for the protection of a distribution transformer, a stacking of varistor of 18.75 cm long is required in a lightning arrester. With the varistors according to the invention which can easily have a break-down voltage of 16 kV/cm or more (see the following detailed description), a varistor with a thickness of 2 cm or a stacking of varistors of 2 cm long will be sufficient to obtain the same protection against over-voltage higher than 30 kV/cm.
The invention and its numerous advantages will be better understood upon reading the following non-restrictive detailed description.
Thus, a first object of the invention is to provide a method for the manufacture of a zinc oxide (ZnO)xe2x80x94and bismuth oxide (Bi2O3)xe2x80x94based varistor having a very high break-down voltage.
This method comprises two first steps, hereinafter called mixing step (a) and milling step (b), which can be combined or inverted.
Step (a) consists in mixing powders of zinc oxide (ZnO) and bismuth oxide (Bi2O3) with one or more other powders of other additives capable of influencing the characteristics of the varistor.
These other additives are preferably selected from the group consisting of metal oxides, carbides, nitrides, nitrates and hydrides that are capable of doping the varistors, modifying the characteristics of their current-voltage curves, modifying the resistivity of phases, reducing their leakage current, increasing their capacity of dissipating energy, controlling their porosity, slowing down the grain growth, increasing their structural integrity, altering the melting points of the phases and increasing their chemical, electrical, mechanical and thermal stabilities. These metal oxides, carbides, nitrides, nitrates and hydrides preferably contains the following elements: Si, Sb, Mn, Ge, Sn, Pb, Nb, B, Al, Ti, Ta, Fe, S, F, Li, Ni, Cr, Mo, W, Be, Br, Ba, Co, Pr, U, As, Ag, Mg, V, Cu, C, Zr, Se, Te and Ga.
In accordance with a particularly preferred embodiment of the invention, the additives that are used are selected from the group consisting of antimony oxide (Sb2O3), manganese oxide (MnO2), alumina (Al2O3), silica (SiO2) tin oxide (SnO2), niobium oxide (Nb2O5) cobalt oxide (CoO or Co3O4), iron oxide (Fe2O3 or Fe3O4) and titanium oxide (TiO2 or TiO). The amount of powders that is used during the mixing step (a) is then preferably selected so that the mixture comprises:
from 0.25 to 10 mol % Bi2O3 
from 1.5 to 4 mol % Sb2O3 
from 0.5 to 4 mol % MnO2 
from 0.00125 to 0.05 mol % Al2O3 
from 0 to 4 mol % of SiO2 
from 0 to 2 mol % SnO2 
from 0 to 2 mol % Nb2O5 
from 0 to 2.5 mol % CoO
from 0 to 2.5 mol % Fe2O3 and
from 0 to 3 mol % TiO2 
the balance consisting of ZnO.
In all cases, it is essential that the mixture be prepared in such a manner that the amounts of powder of zinc oxide present in the mixture be equal to at least 75% mol.
Among the various oxides listed hereinabove, bismuth oxide (Bi2O3) used as a starting material together with zinc oxide (ZnO) is essential to obtain a good insulation between the grains of ZnO and, accordingly, a high varistor effect.
Antimony oxide (Sb2O3) is known to inhibit the grain growth and prevent the transfer of ions in the bismuth-rich liquid phase during the consolidation treatment.
Silica (SiO2) is known to inhibit the grain growth and modify the stability of varistors under continuous electrical constraints.
Manganese and cobalt oxides are known to increase the coefficient a of non-linearity of the varistor and to favorize the interface states.
Iron and niobium oxides as well as the Al3+ cation are also known to increase the coefficient xcex1.
Last of all, titanium oxide (TiO2) is known to increase the size of the grains, which is something that should be avoided in accordance with the invention. However, TiO2 reacts with ZnO to form particles of Zn2TiO4, which seem to increase the nucleation rates and, accordingly, to lead to a much more homogeneous grain size distribution.
The milling step (b) of the method according to the invention is absolutely essential. It consists if subjecting the powders of oxides and/or additives to an intensive mechanical grinding before, during or after their mixing by means of a high energy ball mill in such a manner that the obtained powders be nanocrystalline.
Preferably, this milling step (b) is carried out after the mixing of the powders, that is after the mixing step (a). However, the mixing step can be carried out while the powders are milled, by adding each of the powders one after the other into the ball mill. One can also mill separately each of the powders and thereafter only mix the same.
Thus, for example, the powder of zinc oxide used as a starting material can be milled prior to the mixing step (a), either alone or in combination with doping agents such as Al2O3. In parallel, the powder of bismuth oxide and all the other additives can be mixed, milled and treated at a high temperature equal to or higher than the one of step (c) prior to the mixing step (a).
The milling can be carried out in, for example, a high energical ball mill like those of trademarks SPEX or ZOZ(copyright), having a crucible made of tungsten carbide or chromium steel. Whatever be the equipment that is used, it is essential that the powders contained in the obtained mixture be nanocrystalline.
According to a particularly preferred embodiment of the invention, the nanocrystalline powders that are so prepared are subjected to calcination at a temperature equal to or lower than 550xc2x0 C. This calcination can be carried out on each of the prepared powders when these powders are separately milled. However, the calcination is preferably carried out directly onto the powders after mixing.
After calcination, the mixture can be processed in order to form pellets. This can be achieved by introducing a binder such as polyvinyl alcohol (PVA) into the mixture and subjecting the mixture in which the binder has been introduced to a pressing to form the requested pellets. It must be understood that the mixture may have other forms and thus could be obtained by extrusion or lamination. The powders and PVA can be mixed into a crucible identical to the one of the ball mill for a period of about one hour. The mixture containing the binder can then be pressed under a pressure 500 Mpa or more.
The next step for the method according to the invention is another essential step. This step identified by letter (c) in the xe2x80x9cSummary of the inventionxe2x80x9d and in the appended claims, consists of subjecting the milled and optionally processed mixture to a consolidation treatment including a sintering carried out under temperature and time conditions selected so that the zinc oxide simultaneously keeps the smallest grain size and a low porosity.
The consolidation treatment may also include another treatment consisting of a pressing under different atmospheres (O2, Ar, air, N2, SF6, . . . ), rolling, extrusion, wire-drawing, plasma-spray injection and the like. The treatment preferably involves heating which can be a convection heating, an induction heating, a microwave heating, a laser heating or an electric discharge heating, and which can be carried out either in a continuous manner or for one or several periods of time (rapid thermal annealing, pulse treatment, etc) during or after the consolidation.
According to a particularly preferred embodiment of the invention, the sintering step (c) is carried out in an electric furnace at a temperature lower than 1,200xc2x0 C. for a period of time equal to or lower than 2.5 hours. From a practical standpoint, such a sintering must be carried out at a temperature higher than 800xc2x0 C. to ensure that the bismuth oxide is molten and fully distributed around the zinc oxide grains in order to achieve the requested insulation. However, this sintering must not be carried out at a too high temperature, as such may unduly increase the size of the grains and/or may evaporate some additives.
According to a preferred embodiment of the invention, the sintering is preferably carried out at 1,000xc2x0 C. for a period of time equal to or lower than 1.5 hours.
The heating rate to reach the selected sintering temperature is preferably comprised between 0.5 and 10xc2x0 C./min, the preferred value being 1xc2x0 C./min. Indeed, it has been discovered that the higher is the heating rate, the higher will be the porosity of the obtained varistor, which is something to be avoided.
Last of all, after the consolidation treatment, the obtained pellets can then be cooled at ambient air. As previously indicted, the so obtained varistors have excellent properties.
Thus:
they have a very fine homogenous microstructure and an average grain size of ZnO that is lower than 3 xcexcm and preferably lower than or equal to 2 xcexcm;
they have a break-down voltage higher than 10 kV/cm;
they have a coefficient a of non-linearity of their current-voltage curve higher than 20 and preferably higher than 40 or even 60; and
they have a very small leakage current below the break-down voltage.
The following examples contain the results of tests carried out by the Applicant. Together with the accompanying drawings, these examples will permit to better appreciate the advantages of the varistors according to the invention.
For simplicity""s sake, the varistors prepared in accordance with the invention have been identified as follows in the examples and accompanying drawings:
Sa-b(c)
wherein:
S indicates that the varistor contains silica;
a is the percentage expressed in mol of silica present in the varistor;
b is the sintering temperature; and
c is the sintering time, expressed in hours.
Thus, for example, S2-1,000 (1.5 h) designates a varistor containing 2 mol % of silica, which was prepared by sintering at 1,000xc2x0 C. for 1.5 hours.